1. Field of the Invention
The present invention generally relates to a method and apparatus for preventing degradation in image quality of a photoelectric conversion apparatus due to reception of light from a strong light source.
2. Description of the Related Art
An imaging apparatus such as a digital camera generally includes a photographing optical system and a photoelectric conversion apparatus that photoelectrically converts an object image formed by the photographing optical system. Photoelectric conversion apparatuses can be divided into either passive pixel sensors (PPS) or active pixel sensors (APS). PPSs are non-amplification sensors whose pixels have no function of amplifying electrical charges. PPSs include, for example, a CCD image sensor. APSs are amplification sensors whose pixels have a function of amplifying electrical charges. APSs include, for example, a CMOS image sensor. A CCD image sensor is advantageous for miniaturization and can implement a simple driving method and system configuration. On the other hand, a CMOS image sensor can reduce power consumption and easily speed up the operation. For this reason, a CMOS sensor is suitable for, for example, a digital camera that requires a high operation speed and low power consumption.
When a CMOS sensor receives light as spotlight from a strong light source such as the sun, in particular, the output electric potential difference between rows that have received the light and rows that have not received the light sometimes generates a luminance difference around the spotlight. Additionally, blackening may take place; a part which has been struck by a particularly strong light component of the spotlight blackens. There are various ideas to solve these problems.
Japanese Patent Laid-Open No. 2001-230974 (Paragraph No. 0018; FIG. 2) discloses a technique of solving the luminance difference generated around the spotlight by the output electric potential difference between rows that have received the light and rows that have not received the light. Japanese Patent Laid-Open No. 2001-230974 prevents a load transistor serving as a load of an amplification transistor in a pixel from being turned off by operating a clip means for preventing the electric voltage of a vertical output line from becoming lower than a predetermined voltage in reading the signal level of a pixel that has received strong light. This suppresses the output electric potential difference between rows that have received the spotlight and rows that have not received the spotlight.
Japanese Patent Laid-Open No. 2004-222273 (Paragraph No. 0030; FIG. 2A) discloses a technique of causing a voltage adjusting unit to clip a pixel output signal of reset level and replacing a pseudo signal generated by incidence of strong light with the voltage set by the voltage adjusting unit. The operation principles of the circuitry described in Japanese Patent Laid-Open No. 2004-222273 are the same as in Japanese Patent Laid-Open No. 2001-230974 except that the former executes the clip operation not for the signal level but for the reset level.
However, in the invention of Japanese Patent Laid-Open No. 2001-230974, the set pixel saturation voltage (to be referred to as a “set saturation voltage” hereinafter) needs to sufficiently separate from the set voltage value (to be referred to as a “set clip voltage” hereinafter) of the vertical output line by the clip circuitry, as shown in FIG. 9A. However, when the set saturation voltage should be closer to 0 V, as shown in FIG. 9B, or the power supply voltage should be low, as shown in FIG. 9C, the set clip voltage and set saturation voltage are close to each other. If the set clip voltage and set saturation voltage are close to each other, the set clip voltage and set saturation voltage may reverse, as indicated by circles in FIGS. 9B and 9C, due to threshold variations of the source follower transistor of a pixel or clip circuitry or manufacturing variations of the set clip voltage. When the set clip voltage and set saturation voltage reverse, the apparatus outputs not the set saturation voltage but a predetermined voltage set by the clip circuitry even when the pixel receives strong light. For this reason, even when strong light is uniformly incident on the screen, the pixel exhibits a low-luminance output such as gray, resulting in degradation in image quality.
If the set clip voltage and set saturation voltage are close to each other, the electric potential difference between the gate and source of the source follower transistor of the clip circuitry is small when a pixel outputs a saturation voltage. This increases the subthreshold current in the source follower transistor of the clip circuitry. In this case, the extra current path outside the source follower transistor of the pixel causes the voltage upon saturation to vary as compared to a structure without clip circuitry or a structure in which the set clip voltage is sufficiently separate from the set saturation voltage. The degree of closeness changes between pixels or rows due to manufacturing variations in the threshold value of the source follower transistor of the pixel or clip circuitry. This considerably degrades the image quality; the luminance on the whole screen suffers graininess.
In the invention of Japanese Patent Laid-Open No. 2004-222273, the clip level must be lower than the reset level to prevent blackening in terms of operation principles. However, if they have a large electric potential difference, and the clip level is low, it is unlikely that a decrease in reset level upon incidence of strong light is detected. It is therefore necessary to cause the set clip potential close to the reset level. However, if these potentials are close to each other, the reset level and set clip potential may reverse due to, for example, manufacturing variations, resulting in blackening.